Not Applicable
This invention relates generally to windmills and particularly to an improved sail-type windmill wheel wherein the area of the wheel facing the wind is automatically adjusted.
It has long been established that sail fabric is an economic material for use in the construction of windmill blades. Windmills with sail blades have been in use for hundreds of years along the Mediterranean, and may still be found in use on the island of Crete. These windmills are constructed principally for the purpose of grinding grain or pumping water. These historical designs, however, require constant attention from their operators. Specifically, the frontal area that the sails present to the wind must be reduced in high winds to prevent damage to the sails or windmill structure. A reduction in area results in a reduction of the effective resistance of the windmill wheel to the wind with a corresponding reduction in force upon the wheel. This reduction in sail area is typically accomplished by manually removing the sails or wrapping them around their supporting arm (See Sweney, page 8).
The labor required to operate these historical sail windmills makes them unacceptable for widespread modern use. Inventors have attempted to alleviate this problem through various mechanisms that increase the blade angle of the sails with increasing wind velocity, thereby reducing the area of the sails facing the wind. Blade angle is defined by the angle which an imaginary rigid sail makes with the plane of rotation of the windmill wheel (FIG. 3). The greater the blade angle, the less area the sail presents to the wind. It is desirable, though, to maintain a small blade angle of approximately twelve degrees during operation of the windmill wheel in normal wind speeds so that the sails will work with maximum efficiency (See Calvert, page 81).
U.S. Pat. No. 2,015,777 to Belding (1934) discloses a simple but impractical mechanism for effecting a variable blade angle. Belding""s wind wheel comprises a hub, radially extending arms connected to the hub, and triangular sails secured along one edge to the arms. Belding proposed the use of a simple tension spring interposed between the corner, or in nautical terminology, the clew, of a sail and the next adjacent arm on the windmill. The object of the tension spring is to allow the sail to move outwardly with an increase in wind velocity, thus increasing the blade angle of the sail. I constructed a working sail-type windmill utilizing Belding""s proposed tension springs and discovered that the tension springs were destroyed in high winds, being stretched beyond their elastic limit and permanently deformed. I substituted stronger tension springs for the springs that were damaged in order to find a spring that would not be destroyed in storms. I determined that a tension spring strong enough to survive storms was too strong to allow the sails to assume a satisfactory blade angle (approximately 12 degrees) during normal wind speeds. This is due to the requirement that the tension springs apply a slight tension to the sails at all times; I discovered that the sails, when tethered loosely under the weight of the tension springs, tended to flap destructively with a change in wind direction.
The present invention provides for an improved means of automatically adjusting the frontal area of a sail windmill by adjustment of the blade angle of the sails. I contemplated that instead of a tension spring a compression spring might be used which is acted upon by drawbars. Drawbars are xe2x80x9cUxe2x80x9d shaped metal loops passing through the compression spring""s center from each end and hooking around the opposite end of the spring; when tension load is applied to these drawbars they compress the spring, resulting in an extension of the assembly (FIG. 2). The advantage of this assembly is that the drawbars provide a stop which prevents the spring from excessive deflection and failure. The compression spring in this assembly is not subject to excessive loads; when the spring is fully compressed the travel of the drawbars simply stops. This assembly of a compression spring and drawbars will be referred to as a drawbar extension spring in the remainder of this document.
I constructed a working sail-type windmill with a 12 foot diameter wheel utilizing drawbar extension springs. I observed that the drawbar extension springs both maintained a proper sail blade angle (approximately 12 degrees) during normal wind speeds and provided for a sufficiently large blade angle during high wind speeds to prevent damage to the windmill.
I conducted further experiments which established that a sail-type windmill spinning about a horizontal axis worked with greatest efficiency when the leading edge of the sails are rolled over to form a sleeve wherein the sails are mounted on the spars. I also determined that sails made from high modulus, polyester fiber sailcloth obtained the greatest efficiency. Modulus is the measure of stretch or elasticity of a fabric; a high modulus fabric is a low stretch fabric, unlike the canvas material used on the Mediterranean windmills.
Other more complex mechanisms for the automatic adjustment of sail blade angle are illustrated by U.S. Pat. Nos. 4,066,911, 2,633,921, and 1,417,000. These devices are characterized by complex construction and a high cost to build and maintain. These inventions have not been commercially successful to the best of my knowledge, most likely due to basic inefficiency or a complicated, expensive mechanism.
An improved sail-type windmill wheel is provided in which the area of the wheel facing into the wind is adjusted by an automatic variation in the blade angle of the sails. The wind wheel comprises a hub with a plurality of radially extending arms, substantially triangular sails secured along one edge to said arms, the clew of said sails secured through a sheeting cable to drawbar extension springs, and the drawbar extension springs secured to the tip of the next adjacent arm.
It is accordingly a principal object of the present invention to provide a sail-type windmill wheel in which the blade angle of the sails is automatically adjusted.
It is also an object of the present invention to provide a sail-type windmill wheel which is economical to build and operate.
It is another object of the present invention to provide a sail-type windmill wheel in which the blade angle of the sails increases with increasing wind velocity.
It is a further object of the present invention to provide a sail-type windmill wheel in which the blade angle of the sails is maintained at approximately 12 degrees or less during normal wind speeds.
It is also an object of the present invention to provide a sail-type windmill wheel in which the leading edge of the sails are rolled over to form a sleeve wherein the sails are mounted on a circular spar.
Yet another object of the present invention is to provide a sail-type windmill wheel in which the clew of each sail is secured by cordage that has a breaking strength less than that of the sails, sheeting cables, drawbar extension springs, or any other linkage securing the sails, so that such cordage shall break in hurricane winds thus allowing the sails to extend normal to the plane of rotation of the wind wheel.
One further object of the present invention is to provide sail-type windmill wheel in which the sails are constructed of high modulus, firm finish, polyester sailcloth.
These and other objects of this invention will be more clearly understood from the following detailed description when taken in conjunction with the drawings wherein.